1. Field of the Invention
The present invention relates to brazing alloys, and more particularly, to Ni (nickel)--Pd (palladium)--Cr (chromium)--B (boron) brazing alloys suitable for brazing in the 1800.degree.-2000.degree. F. temperature range.
2. Description of the Prior Art
Application of various metals and their alloys for use as structural or operational components in high temperature operating systems such as turbomachinery for use in gas turbine engines often requires joining to form a sound, structural joint. Joining can be accomplished in a number of ways, the more common and effective being welding or brazing.
During welding, a portion of the members being joined are melted and resolidified. A weld joint can have a grain structure significantly different from the material joined and will generally have reduced mechanical properties as a result of the melting and solidification. However, during brazing, a metal or alloy generally referred to as the brazing alloy, is placed between closely fitted members to be joined, and heated to a temperature, generally referred to as the brazing temperature, sufficient to cause melting of the brazing alloy but not the alloy of the members. The brazing alloy subsequently is resolidified during cooling. A bond results principally from the combination of heat and interdiffusion of the brazing alloy with the alloy of the structural members being joined. The brazing alloy is selected to provide a sound bond which results in optimum high temperature mechanical properties across the joint.
Various factors are considered in the development, selection, and application of a brazing alloy. These factors include processing, service conditions, physical and mechanical properties of the brazing alloy and economic considerations. These factors are well known and practiced in the art.
Many aircraft gas turbine components are fabricated using brazing techniques. Most generally, the nickel-base superalloys or various high strength steels are selected as the structural materials of such components. A number of materials thus selected require that brazing be performed in the temperature range of from about 1800.degree. F. to about 2000.degree. F. to provide sound joints while maintaining high temperature mechanical properties.
One high-temperature nickel-base superalloy currently used as brazed structural members in high-temperature applications is an age-hardening alloy, commercially available as Inconel 718 alloy, and having a nominal composition of about 19% Cr, 5% Cb (columbium)+Ta (tantalum), 3% Mo (molybdenum), 0.6% Al (aluminum), 0.8% Ti (titanium), 53% Ni, with balance Fe (iron) and incidental impurities. As used herein, all percentages are weight percentages unless otherwise noted. Such alloy has a solution temperature in the range of 1700.degree.-1850.degree. F. This alloy, however, is susceptible to excessive grain growth when exposed to temperatures as high as about 2000.degree. F., resulting in a significant reduction of mechanical properties. To avoid such grain growth and loss of properties, it is desirable to use a brazing alloy which has a brazing temperature of about 1900.degree. F. or below.
Brazing alloys used previously within a brazing temperature range of about 1800.degree.-2000.degree. F. have included the gold-bearing brazing alloys, such as, for example, 82% Au (gold), 18% Ni (hereinafter referred to as 82-18), and 20.5% Au, 66.5% Ni, 5.5% Cr, 2.2% Fe, 3.5% Si (silicon), 2.1% B (hereinafter referred to as Au-6). These brazing alloys have application limitations based on their strength capability, their service temperature limit, and their ductility. The large amounts of gold present in these alloys results in very high cost which makes their selection and use unattractive.
Other, less expensive brazing alloys available for brazing in the temperature range of 1800.degree.-2000.degree. F. include those alloys which contain substantial amounts of silver, titanium manganese, copper or phosphorus. Such brazing alloys are not attractive for use in high-temperature gas turbine applications for various reasons. For example, silver-containing brazing alloys are very corrosive to nickel-base alloys at high temperatures experienced in aircraft gas turbines. Titanium-containing brazing alloys lack high temperature service capabilities. Manganese and copper-containing brazing alloys exhibit poor oxidation resistance above about 1000.degree. F. Finally, the phosphorus-containing nickel-base brazing alloys produce excessive structural metal erosion and joints which are excessively brittle. Such brazing alloys have limited usefulness for high temperature applications.
Accordingly, a need exists for a less expensive, gold-free brazing alloy suitable for brazing certain high-temperature structural materials in the same temperature range as the previously discussed gold-containing brazing alloys. Additionally, the improved brazing alloy must have a better combination of strength and ductility as well as comparable or higher operating service temperature than currently available brazing alloys.